Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
Field of the invention
.
This invention relates generally Jo tracking error
detecting apparatus for an optical head end more particularly,
is directed to a tracking error detecting apparatus for an
- optical head for use with an optical recording apparatus, an
optical reproducing apparatus, an optical recording and/or
reproducing apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a diagram showing A construction of an
optical system of a prior art optical head;
c
Fig. 2 is a front view of a prior art semiconductor
laser apparatus;
it. 3 is a diagram useful for explaining returned
beams of the prior art semiconductor laser apparatus;
Foe 4 is a characteristic graph of interference;
Fig 5 is a diagram useful for explaining an
occurrence of the interference; and
Fox to 15 are respectively diagrams showing
embodiments of a wracking error detecting apparatus according
to the present invention.
Description of the Prior Art
Fig. 1 is a diagram showing an example of a prior
art tracking error detecting apparatus for an optical head, in
which reference letter OH generally designates an optical head
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on the whole. In Fig. 1, reference numeral 1 designates a
semiconductor laser apparatus using a laser diode. In this
semiconductor laser apparatus 1, a divergent laser beam L of
ellipse-shape in cross section oscillated out or emitted from
a laser light emission end face lo thereof is made incident on
a collimator lens 2 (which may be omitted as required) in which
', it is collimated as a parallel beam. This parallel beam is
to then made incident on a diffraction grating 3. From the
diffraction grating 3 there are produced a zero-order beam Lo
and + first-order beams Lo and Lo (beams higher than second-
-, order or lower than - second-order are neglected). These beams
-I are traveled through a non-polarizing beam splitter (half mirror)
4 (if a polarizing beam splitter it. used, a 1/4 wavelength plate
is provided between it and an objective lens 5) and is then
I, 15 made incident on the objective lens 5 thereby converged.
¦ The converged zero-order beam Lo and + first-order beams Lo
I, and Lo are made incident on a recording surface of an optical
3 disc 6 used as an optical recording medium (including a magneto-
i optical recording medium) with a predetermined spacing (for
example, loom) there between.
The zero-order beam Lo and first-order beams Lo
and Lo reflected by the optical recording disc 6 are traveled
through the objective lens 5 and introduced into the beam
splitter 4 t while a part of them is reflected on a reflecting
surface pa of the beam splitter 4 and thereby made incident on
a photo-detector 7. The photo-detector 7 is formed of three
photo-detecting sections to permit the zero-order beam Lo and
+ first order beams Lo and Lo to become incident thereon
i. .
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separately.
In the case of a tracking error detecting method
known as a so-called three spots method, by calculating a
difference between a pair of photo detected outputs from a
pair of photo-detecting sections on which the first-order
beams Lo and 1_1 are incident, it is possible to obtain a
tracking error signal which is corresponding to the tracking
state of the zero-order team Lo on the recording surface of the
optical disc 6. From the photo-detecting section on which the
zero-order beam Lo is made incident, there are produced a no-
produced signal, focusing error signal and 80 on.
urethra there US a etude Lee Pi potent ply
cation publ~slled under No. KIWI which is the improvement
of a tracking error signal detecting method eased on a so-
called push-pull method. In this known method, of three beams,
zero-order beam and one of the side beams provided at the both
sides of the zero-order beam or all of three beams are used.
More specifically, this method uses such a
photodetector for the three beams the photo-detecting section
of which is divided into equal two sections. Further, it is
arranged such that when the beam spot by the zero-order beam
lies on the track of the optical disc, the beam spots by the
side beams provided at both sides of the zero-order beam lie
on the lands or they are formed with a displacement of 1/2
track pitch. Accordingly, the outputs based on the difference
between the detected outputs from the respective photo-detect-
in sections of the photo-detector relative to the beam spots,
or the push-pull outputs become opposite in phase with respect
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to the beam spots formed by the zero-order beam and + first-
order beams. Whereas, the Dock fluctuation components produced
in the push-pull outputs by the lateral displacement of the
objective lens and the skew of the disc become same in phase.
Accordingly, if a difference between the push-pull
output PP0 from the photo-detector relative to the zero-order
beam and the push-pull output PP1 or PP2 from the photo-detec-
ion for first-order beam or - firs order beam is calculated,
regardless of the lateral displacement of the objective lens
and the skew of the optical disc, it is possible to obtain a
tracking error signal which has no DO fluctuation components.
I Using three push-pull outputs, PP0, PP1 and PP2, it
! is also possible to obtain the tracking error signal by
calculating PP0 - (Go PP1 Go PP2). In this case, Go and Go
represent constants which are presented by considering a dip-
furriness between the n5-of-:~be ~h~to-detectors.
Next, an example of the semiconductor laser apparatus
1 will be described with reference to Fig. 2.
Referring to Fig. 2, this semiconductor laser
apparatus 1 is generally fixed to a header portion 8 made of
metal such as copper and so on serving as one electrode which
becomes a heat sink. In other words, in this example, the
header portion 8 is formed of only the heat sink.
The structure of the laser chip of the semiconductor
laser apparatus 1 will be described in the order of the upper
layer to the lower layer. Reference numeral lo designates an
electrode layer, lb on n-GaAs layer (substrate layer), to an
n-Gal_yAQ~As layer (cladding layer), id a Gal_xAQxAs layer
,
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(active layer), to a p-Ga1_yAQyAs layer (cladding layer and
if p-GaAs layer. The above laser beam L is oscillated out
from the active layer id. If the laser beam emission end face
(wall surface) lo of this semiconductor laser apparatus 1 is
taken axis a front, the width thereof is in a range from 100 to
300~m, the height (thickness) thereof is in a range from 80 to
loom and the depth thereof is in range from 200 to 300~m,
I, respectively. The height of the active layer id from-the upper
surface of the header portion 8 is several em.
lo By the way, in practice, when the tracking error
detecting method of not only the three spots method but also
improved push-pull system is used, if in the optical disc,
there is the skew in the tangential direction thereof, a DO
fluctuation is produced in the tracking error signal so that
it is not possible to detect the tracking error precisely.
¦ After various researches, the present inventors et
at. have studied the following causes.
Referring to Fig. 1, the zero-order beam Lo and the
! + first-order beams Lo and Lo reflected on the optical disc
2Q 6 are traveled through the objective lens 5 and then reflected
on the reflecting surface pa of the beam splitter 4. Also
they are traveled through the beam splitter 4 and introduced
islet the diffraction grating 3 which produces the corresponding
zero-order beam Lo and first-order beams Lo and Lo
separately. Then, they are traveled through the collimator
lens 2 and are introduced into the semiconductor laser apparatus
1. The beam amount of the laser beam to be incident on the
semiconductor laser apparatus 1 is large when the non-polarizing
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beam splitter is used and small when the polarizing beam
splitter is used. In this case, in accordance with the position
of the relative rotation angle between the laser beam emission
end face lo of the semiconductor laser apparatus 1 and the
diffraction grating 3, there are cases that a central beam La
and side beams Lb and lo at the both sides of the central beam
La to be incident on the semiconductor laser apparatus 1 are
respectively arranged such that as shown in Fig. 3, the central
beam La is positioned in the active layer id on the end face
¦ 10 lo and the both side beams Lb and lo are positioned in the up
I and down direction on the straight line passing through the
¦ central beam La and perpendicular to the active layer id,
that the central beam La and the both side beams Lb and lo are
all positioned in the horizontal direction positioned on the
active layer id and that the straight line formed by connecting
the central beam La and the both side beams Lb and lo is
positioned at a desired angle position which is intermediate
¦ between the above-described two cases. The central beam La and
lo are provided by diffracting again the zero-order beam Lo and
+ first-order beams Lo and Lo by the diffraction grating 3
and superposing them in a mixed state.
When at least one of the both side beams Lb and lo
is made incident on the plane of the header portion 8, the plane
of the header portion 8 is formed rough so that the beam is
reflected irregularly. On the other hand, when at least one
of the both side beams Lb and lo is made incident on the laser
beam emission end face lo of the semiconductor laser apparatus
1, the end face lo is excellent in reflectivity for example,
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10%) so that the beam is reflected on this end face lay As
described above, the zero-order beam and the + first-order
beams incident on the semiconductor laser apparatus 1 are
reflected thereon, diffracted again by the diffraction grating
3 and reached to the optical disc 6 so that on the photo-
detector 7, there is formed a complicated intexfer nice pattern.
the interference pattern is varied by a difference
(phase difference) between the lengths of optical paths of the
zero-order beam and the + first-order beams. Thus, the inter-
furriness pattern is varied by the change of the skew angle of
the optical disc 6. Accordingly, the tracking error signal is
varied by the change of the skew angle I of the optical disc
6 so as to have a periodicity as, for example, shown in Fig. 4.
In practice, as ¦ ¦ is increased, the level of the tracking
error signal So is attenuated. When the hot side beams Lb and
lo are both made incident on the laser beam emission end face
lay the amplitude of the waveform corresponding to Fig. 4 be--
comes twice the amplitude shown in Fig. 4 and the phase thereof
becomes different from the phase of Fig. 4.
Next, the analysis of the above-described inter-
furriness pattern will be made with reference to Fig. 5 (from
which the lens system is omitted).
Referring to Fig. 5, lo shown by a solid line design
notes the laser beam emission end face which is, however, is
inclined relative to the laser beam emission end face lo post-
toned at the normal position shown by a broken line.
Further, reference numeral 6 shown by a solid line designates
an optical disc. This optical disc 6 has a skew and hence is
Lo
inclined relative to the normal position shown by a broken line.
The zero-order beam Lo is perpendicular to the laser beam ems-
soon end face lo at the normal position and the recording sun-
face of the optical disc 6 at the normal position. design
notes an angle of the first-order beam Lo relative to the
zero-order beam Loo Al designates an optical path length
between the laser beam emission end face lo and the diffract
lion grating 3. Q2 designates an optical path length between
the diffraction grating 3 and the recording surface of the
optical disc I clue and AWOKE respectively designate optical
path length differences between the optical path length Q1 and
Q2 of the zero-order beam Lo and + first-order beam Lo
~Q3 and I respectively designate an optical path difference
caused by the skew of the optical disc 6 and an optical path
difference caused by the skew of the laser beam emission end
face lay
Further, g is taken as a phase difference between
the zero-order beam Lo and 1- first-order beam Lo in the
diffraction grating 3. to and if are respectively taken as
transmittances of the zero-order beam Lo and the + first-order
beam Lo in the diffraction grating 3. Reference letter t is
taken as the transmittance of the half mirror 4 and r and f are
respectively waken as reflectivities on the recording surface
of the optical recording medium 6 and on the laser beam ems-
soon end face lay
Then, a complex amplitude of light at a point A on the recording surface of the optical disc 6 at which the +
first order beam Lo is incident will be considered separately
it 3
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,
with respect to the following four cases
(1) a : first-order beam Lo is made incident at the point
A, directly.
(2) a : zero-order beam which results from reflecting the
zero-order beam Lo on the optical disc 6 and introducing it
again into the diffraction grating 3 is reflected on the laser
beam emission end face lo and again introduced into the defy
fraction grating 3 to thereby produce the + first-order beam
Lo This + first-order beam Lo is then incident at the point
A.
(3) a : + first-order beam is provided by reflecting the zero-
order beam Lo on the optical disc 6 and introducing it again
into the diffraction grating 3. This + first order beam is
reflected on the laser beam emission end face lo and incident
again on the diffraction grating 3 to thereby produce the zero-
order beam. This zero-order beam is then made incident at the
point A.
(4) a : ~ero-order beam is provided by reflecting the + first-
order beam Lo on the optical disc 6 and introducing it again
into the diffraction grating 3. This zero-order beam is no-
floated on the laser beam emission end face lo and introduced
again into the diffraction grating 3 to thereby produce the
zero-order beam Then, this zero-order beam is made incident
at the point A
The above-described a to a will be expressed by the
following equations.
a t exp {i Al+ g + I ~Q2+ ~Q3) }
. . I
, I..
I .,'~
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~7g~ .
a =i2ilt3rf'exp [j {3 ( Al Q2) 9 + ~Q2
~Q3}] --(2)
a i2qilt3rf exp [i {3 (Q1 + Q2) t g 2~Q1
~Q2 ~Q3 + 2~Q4}3 -. (3)
a = i2ilt3rf'exP [i {3 (Al + Q2) g
3 ~Q2 + ~Q3~ + 2~Q1 + 2~Q4}] ... (4)
In order to simplify the calculations, if the length by
which the laser beam can beinterferen is selected to be
lower than 2 (Q1 + R2)~ the intensity IA of light at the
point A is expressed by the following equation (5).
PA I a I + ¦ a + a a ¦ 2
- islet [1 + i40t4r2f2 {3 + Casey 2 (~Q1 + ~Q4)
coy 2 (~Q1 + ~Q4 + ~Q2 + ~Q3) +
Casey 2 (~Q2 ~Q3)}] -- (5)
Further, when the both side beams Lb and lo are both made
incident of the laser beam emission.. end face lay if the
first-order beam Lo is made incident at the point A on
the recording surface of the optical disc 6 and the - first-
order beam Lo becomes incident on a point B which is sum-
metrical with respect to the zero-order beam Lo the intensity
It of the light at the point A is given as by En. (5), while
an intensity IT of light at the point B is expressed by the
following equation (6).
IT = iota [1 + iot4r2f2l3 + Casey 2 (clue
_ I + Casey 2 (~Q1 ~Q4 +
2 ~Q3) + Casey 2 (~Q2 ~Q3)}] ... (6)
As described above, the complex interference pattern
is produced on the photo-detector 7. Particularly, when
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relative to the central beam La, the both sides beams Lb
and lo are arranged in the vertical direction and the side
beam Lb becomes incident on thy laser beam emission end face
lo and the side beam lo becomes incident on the header portion 8,
respectively, the side beam Lb is reflected on the laser
beam emission end face lo and the side beam lo is reflected
irregularly on the header portion $ (whose surface is formed
rough so that with respect to the both side beams Lb and lo
which are returned to the semiconductor laser apparatus 1 and
then incident on the side of the disc 6 again there is caused
an unbalance, thus the DO fluctuation being produced in the
tracking error signal. Similarly the DO fluctuation is
produced in the tracking error signals of the three spots
method and the push-pull method as mentioned before.
OBJECTS AND SUMMARY OF THE INVENTION
:
Accordingly, it is an object of this invention to
provide a tracking error detecting apparatus in which a
tracking error signal indicative of an error of a zero order
beam from an optical head to the track of an optical recording
medium can be prevented from being affected by the inclination
of the optical recording medium in the track direction, or the
variation of the tangential skew angle thereof.
It is another object of this invention to provide a
tracking error ducking apparatus which is suitable for use
with an optical recording apparatus, an optical reproducing
apparatus, an optical recording and/or reproducing apparatus
,,,
.,
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and so on.
According to one aspect of the present invention,
there is provided a tracking error detecting apparatus for
an optical head comprising a semiconductor laser apparatus,
a diffraction grating, a beam splitter, an objective lens
and a photodetector in which a laser beam from said
semiconductor laser apparatus is divided into a main beam
and two side beams at both sides of said main beam by said
diffraction grating, said three beams are introduced through
¦ lo said beam splitter and said objective lens to an optical
recording medium, beams reflected on said optical recording
I medium are introduced through said objective lens to said
beam splitter and reflected thereon, said reflected beams
are made incident on said photo-detector and detected outputs
1 15 of at least two beams of said three beams from said photo-
¦ detector are used to produce a tracking error signal
corresponding to the tracking state of said main beam on said
optical recording medium, said tracking error detecting
apparatus far an optical head being characterized in that the
reflection of said side beam of said reflected beams on said
: optical recording medium which passes through said objective
lens, said beam splitter and said diffraction grating and
: returns to said semiconductor laser apparatus is suppressed
at a laser beam emission end face to thereby reduce influence
by the skew of the optical disc on the tracking error signal.
These and other objects, features and advantages of
: the present invention will become apparent from the following
detailed description of the preferred embodiments taken in
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conjunction with the accompanying drawings, throughout which
like reference numerals designate like elements and part.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring initially to Fig. 6, an embodiment of a
tracking error detecting apparatus according to the present
invention will hereinafter be described in detail, in which
case the laser beam emission end face lo of the semiconductor
10 . laser apparatus or element 1 it high in reflectivity (for
example, 10~). In Fig. 6, like parts corresponding to those
of Figs. 1 to 5 are marked with the same references and will
not be described.
Fig. 6 shows a case in which the straight line
15 . formed by connecting the central beam La and the both side
beams Lb and lo each of which is to be incident on the
semiconductor laser element 1 is made substantially perpendicular
to the active layer id of the semiconductor laser element 1.
By the way, in this embodiment, the thickness Height) of the
2Q semiconductor laser element 1 is selected to be as thin (low)
as about 80~m so that of the both side beams Lb and lo, the
side beam Lb, which is shown on the upper side of Fig. 6,
can be prevented from being map incident on the laser beam
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emission end face lay In Fig. 6, the beam lo at the lower side
becomes incident on the heat sink 8, on which it is irregularly
reflected. Other arrangements are similar to those of
Figs 1 to 5.
In consequence, according to the optical tracking
error detecting apparatus with the laser semiconductor
apparatus of this invention, even if the beams reflected on the
optical recording medium are traveled through the objective
lens 5, the beam splitter 4 and the diffraction grating 3
and then introduced to the semiconductor laser element l,
there is no fear that the beams will be reflected on the laser
beam emission end face lay Thus it is possible to obtain a
tracking error signal of the zero-order beam from the optical
head Owl relative to the optical recording medium which can be
prevented from being varied due to the variation of -the
tangential skew angle of the optical recording medium.
If there arises a problem of strength of the semi-
conductor laser element 1 when it is reduced in thickness,
as shown in Fig. 7, a groove 9 is formed from the upper surface
of the semiconductor laser element 1 at its central portion
so that the upper laser beam Lb can be prevented from being
made incident on the laser beam emission end face lay
If there is a possibility that the both side beams
Lb and lo will become incident on the active layer id on
the laser beam emission end face lay the width of the semi-
conductor laser element 1 is reduced or the grooves are formed
at both sides of the semiconductor laser element 1 in order
to avoid such possibility.
Referring to Fig 8, a second embodiment of the
optical tracking error detecting apparatus according to the
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present invention will be described, in which case, the
laser beam emission end face lo of the semiconductor laser
element 1 is high in reflectivity (for example, 10%).
In Fig. 8, like parts corresponding to those of Figs. 6 and 7
are marked with the same references and will not be described.
In this embodiment, as shown in Fig. 8, a part of
the heat sink 8 is extended to form a light shield plate
(member) 9' so as to oppose the laser beam emission end face
lo of the semiconductor laser element 1 with a predetermined
spacing there between through which a window pa is formed
at its portion corresponding to the laser beam emitting portion
of the active layer or region id whereby to secure the emission
of the laser beam L.
Then, according to -the tracking error detecting
lo apparatus of such optical head, even if the laser beams
reflected on the optical recording medium are traveled through
the objective lens 5, the beam splitter 4 and the diffraction
grating 3 and then incident on the semiconductor laser element
l, such laser beams are reflected irregularly by the light
shield plate 9' so that it is possible to obtain a tracking
error signal which can be prevented from being varied due to
the change of the ~ero-order beam from the optical head OH
by the tangential skew angle of the optical recording medium.
If there is a possibility that the both side beams
Lb and lo will become incident on the laser beam emission end
face lay as shown in Fig 9, a part of the heat sink 8 is
extended at two places to form a pair of light shield plates
9' and 9' which are opposed to the laser beam emission end
face lo of the semiconductor laser element 1 with a predetermine
spacing there between. The pair of the light shield plates 9'
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and 9' are disposed at the positions to shield the both side
beams Lb and lo which are to be incident on the semiconductor
laser element 1.
While it may be possible that the light shield
plate 9' for absorbing or shielding the light can be bonded
to the laser beam emission end face lo of the semiconductor
laser element 1 by bonding and the like, in this case an
insulating material is used in this case.
Figs. 10 and 11 are each a diagram showing a third
embodiment of the tracking error detecting apparatus according
to the present invention. This third embodiment is further
improved such that as shown in the first or second embodiment,
the upper beam Lb can be prevented from being made incident
on the laser beam emission end face or the laser beam emission
end face of the semiconductor laser is low in reflectivity
(for example, about 2 to I or a laser diode of high output
is used. In Figs. 10 and 11, reference numeral 10 designates
a laser diode chip and 11 a header portion made of metal, lo
a laser beam emission end face and lo an active layer,
respectively.
In this embodiment, the surface of the header portion
11 at which the side beams traveled through the collimator
lens 2 along -the optical paths shown by broken lines are
incident is coated with a non-reflecting coating material 12.
Accordingly, even if the side beam is made incident on the
header portion 11 at its portion coated with the non-reflecting
coating material 12, the reflection thereof can be suppressed
so that the side beam can be prevented substantially from
becoming incident on the collimator lens 2 once again.
In this case, it is not necessary to coat the
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non-reflectirlg coating material 12 on the whole area of the
header portion 11 at its surface opposing to the collimator
lens 2 in the lateral direction but it is possible to coat
it on only its portion under the laser diode chip 10 on
which the laser beam is made incident.
Further, the positions of end sides AYE and 12B in
the right and left directions of the portion coated with the
non-reflecting coating material 12 in Fig. 11 can be used as
markers for matching the positions of the laser diode chip 10
lo and the header portion 11 when the laser diode chip is mounted
on the header portion. In other words, if the distance between
the both end sides AYE and 12B is selected to be equal to the
width of the laser beam emission end face lo of the laser
diode chip 10, the positions can be matched by only matching
lo the both side ends of the laser diode chip lo with the both
end sides AYE and 12B of the non-reflecting coating material-
coated portion 12. It is needless to say that the both end
sides AYE and 12B are both not used as the markers but it is
sufficient that one of them is used as the marker for matching
the positions.
The semiconductor laser apparatus 1 is not limited
to such structure that the laser diode chip 10 is directly
mounted on the heat sink made of metal but may be such one as
shown in Fig. 12 in which the laser diode chip 10 is mounted
on a silicone substrate 13 and then this silicone substrate 13
on which the laser diode chip lo is mounted is mounted on a
heat sink 14. In other words, in this easel the header
portion is formed of the silicone substrate 13 and the heat
sink 14.
In the case of the semiconductor laser apparatus
I
with such structure, there is a case that the side beam will
become incident not on the heat sink 14 but on the silicone
substrate 13.
Therefore, in the case of the example of the semi-
conductor laser apparatus shown in Fig. 12, the non-reflecting
coating material 12 is coated on the necessary portions of the
silicone substrate 13 and the heat sin 14.
Fig. 13 is a diagram showing other embodiment of the
optical tracking error detecting apparatus according to this
lo invention, in which another embodiment of the semiconductor
laser apparatus is used. As shown in Fig. 13, reverence
numeral lo designates a laser diode chip and 11 a header
portion made of metal.
In this embodiment, as shown in Fig. 13, the plane
of the header portion 11 on which the side beams, which are
passed through the collimator lens 2 along the optical paths
shown by a broken lines, are incident is inclined as shown
in the figure at US so that the reflected side beams thereon
can be prevented from becoming incident on the collimator lens
2 once again.
In this case, if the surface of the inclined portion
US is not formed as a rough surface but as a mirror surface
so as to avoid the irregular reflection, there can be achieved
a significant effect.
Further, as shown in Fig. 14, if Go is taken as
an angle between the main beam and the side beam, an NOAH.
(numerical aperture) of the collimator lens 2 is taken as Nay
and the condition of c = sin~lNAc and an angle oh between the
inclined portion US and the plane perpendicular to the main
beam is selected to satisfy the following condition
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oh + Go I '
the reflected light of the side beam from the header portion
11 can be prevented from being made incident on the collimator
lens 2.
In this case, it is not necessary to provide the
inclined portion US on the whole area of the header portion
11 at its surface opposing to the collimator lens 2 in the
lateral direction. It is, however, sufficient that as shown
in Fig. 15, on the header portion 11, a concave portion 14
having the inclined portion US is provided only at the
incident portion of the side beam under the laser diode chip
10. In this case, both sides AYE and 14B of this concave
portion 14 can be used as markers for matching the positions
in the lateral direction when the laser diode chip 10 is
mounted on the header portion 11. In other words, if the
distance between the both sides AYE and 14B is selected to
be equal to the width of the end face lo of the laser diode
chip 10, the matching of the positions there between can be
carried out only by matching the bath sides of the laser
diode chip 10 with the both side edges AYE and 14B of the
concave portion 14. Of course, it is possible that the both
side edges AYE and 14B are used as the markers but either
of them is used as the marker for matching the positions.
The inclined portion US may not be formed recta-
linearly but be formed as a curved surface.
According to these embodiments of the invention as
set forth above, since of the side beams the reflected beam
of the beam incident on -the header portion can be prevented
from being returned to the optical system again, if it is
arranged to suppress the reflected beam at the side of the
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latex beam emission end face of the laser diode chip, it is
possible to remove the DO fluctuation from being produced
in the tracking error signal by the skew of the optical disc
more effectively.
The above description is given on the preferred
embodiments of the invention, but it will be apparent -that
many modifications and variations could be effected by one
skilled in the art without departing from the spirits or
scope of the novel concepts of the invention, so that the
scope of the invention should be determined by the appended
claims only.
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